Last week, a juror in a big federal drug trial in Florida admitted to the judge that he had been doing research on the case on the Internet, directly violating the judge’s instructions and centuries of legal rules. But when the judge questioned the rest of the jury, he got an even bigger shock.

Eight other jurors had been doing the same thing. The federal judge, William J. Zloch, had no choice but to declare a mistrial, wasting eight weeks of work by federal prosecutors and defense lawyers.

“We were stunned,” said the defense lawyer, Peter Raben, who was told by the jury that he was on the verge of winning the case. “It’s the first time modern technology struck us in that fashion, and it hit us right over the head.”

It might be called a Google mistrial. The use of BlackBerrys and iPhones by jurors gathering and sending out information about cases is wreaking havoc on trials around the country, upending deliberations and infuriating judges.

Last week, a building products company asked an Arkansas court to overturn a $12.6 million judgment against it after a juror used Twitter to send updates during the civil trial.

And on Monday, defense lawyers in the federal corruption trial of a former Pennsylvania state senator, Vincent J. Fumo, demanded that the judge declare a mistrial after a juror posted updates on the case on Twitter and Facebook. The juror even told his readers that a “big announcement” was coming Monday. But the judge decided to let the trial continue, and the jury found Mr. Fumo guilty. His lawyers plan to use the Internet postings as grounds for appeal.

Jurors are not supposed to seek information outside of the courtroom. They are required to reach a verdict based only on the facts that the judge has decided are admissible, and they are not supposed to see evidence that has been excluded as prejudicial. But now, using their cellphones, they can look up the name of a defendant on the Web, or examine an intersection using Google Maps, violating the legal system’s complex rules of evidence. They can also tell their friends what is happening in the jury room, though they are supposed to keep their opinions and deliberations secret.

A juror on a lunch or bathroom break can find out many details about a case. Wikipedia can help explain the technology underlying a patent claim or medical condition, Google Maps can show how long it might take to drive from point A to point B, and news sites can write about a criminal defendant, his lawyers or expert witnesses.

“It’s really impossible to control it,” said Douglas L. Keene, president of the American Society of Trial Consultants.